Heat treating magnesium base alloys



Filed Feb. l2, 1951 BR/NELL HARDNESS YIELD Por/v o L25 .ma/.- 0 4L a 1216 zo Z4 300. T/ME -HOU/es INVENToRs ATTORNEY Patented Nov. 2l, 1933UNIT-ED STATES A 1,936,550 HEAT TaEATmG MAGNESIUM BASE ALLoYs John A.Gann and Leo B. Grant, Midland, Mich., assignors to The Dow ChemicalCompan Midland, Mich., a corporation of Michigan Application February12, 1931. Serial No. 515,236 5 Claims. (Cl. 14S-21.3)

The present invention, relating as indicated, to the treatment of lightmetal alloys, has more particular regard tothe treatment of alloys inwhich magnesium is the predominant constituent. The

5 specific alloys involved are magnesium-manganese alloys wherein themanganese may be the only metal alloyed with magnesium or may be onlyone of the metals alloyed therewith. It is known that articles made frommagnesium alloys, wherein 'the magnesium constitutes 80 per cent andupwards of the total, possess valuable characteristics when in the formof cast articles or the like. The corrosion resistance of binarymagnesium-manganese alloys is the best that has been obtained withmagnesium alloys, and moreover the workability of this alloy makes ithighly desirable for many uses. The binary magnesium-manganese alloys incast form, however, have a relatively low yield point and a somewhatlower hardness than most of the commercial magnesium alloys. Thesedisadvantages have to a certain extent limited the commercial use of thealloys in question. At this time it was known that castings made frommagnesium-manganese alloys contained manganese in solid solution. Ifsome of this manganese were present in solid solution in excess of itsequilibrium solubility, it appeared to us that part or all of suchexcess might be precipitated by heat treating, and advantageously affectthe properties of the alloy article treated. Accordingly, we heattreated some high percentage manganese alloys for 24 hours at varioustemperatures, for example 150 and 200 C., according to the conventionalprecipitation practice for heat treating magnesiumaluminum andmagnesium-zinc alloys. Hardness and yield point tests on the alloys heattreated at 150 and 200"` C. showed that there was apparently noprecipitation of manganese at these temperatures, there being noproperty change at 150 C. and the improvement made, if any, at 200 C.was Within the limits of experimental error. We have, however, foundthat manganese can be precipitated and that the temperature for itsprecipitation in magnesiummanganese alloys is unexpectedly high, in factit is even above the maximum solution heat treatment temperature forbinary magnesiumzinc alloys. We have further discovered that a magnesiumalloy containing not only manganese in solid solution in excess of itsequilibrium solubility, but another metal capable of going intosolution, may be heat treated so as to simultaneously precipitate themanganese and cause the other metal to pass into solution. As a resultof the foregoing discoveries,'it is no longer necessary to be limited tohaving the manganese present in the casting or like article in solutionin excess of its equilibrium solubility, but much better properties canbe obtained by precipitating part or all of such excess manganese by asultable heat treatment.

Having regard tothe accomplishment ofthe foregoing and related ends, theinventionfthen', consists of the steps and product vhereinafter fullydescribed and particularly pointed out in the claims, the annexeddrawing and the following description setting forth but several of thevarious ways in which the principle of the invention may be used.

In said annexed drawing:-

The figure -is a diagram illustrating the changes in yield point andhardness of a binary magnesium-manganese alloy during heat treatment atdilferent temperatures over varying periods of time according to ourmethod or process.

As indicated above, the present improved process has to do with lightmetal alloys in which magnesium is the predominant ingredient. The alloymust furthermore have manganese present 8'0 in solid solution in excessof its equilibrium solubility at normal temperatures, i. e., in excessof about 0.5 per cent. In addition to manganese, the alloy may containother metals, part of which may or may not be capable of being dissolvedin the alloy or otherwise changed in state by solution heat treatment atthe precipitation heat treatment temperature for manganese. The placingof such material in solution in the alloy or otherwise changing itsstate simultaneously with the precipitation of manganese therein isaccordingly one phase of the present invention. What we accomplish byour present improved process is to regulate the length of time andtemperature of heat treatment of such an alloy so that the desiredamount of precipitation of manganese as such takes place with resultingincrease in hardness and yield point, and, where other ingredients arealso present that are capable of benecial change of state at thetempera- 100 ture range involved, the temperature and duration 'of heattreatment can be so regulated that both beneficial effects aresimultaneously obtained. While in the drawing we have illustratedv theeiect of precipitation heat treatment on a 105 binarymagnesium-manganese alloy, it will be understood that the process may beadvantageously applied to more complex magnesiumalloys having manganesepresent in solid solution in excess of its equilibrium solubility, andthat part of 110 the other constituents may be simultaneously changed instate, during the precipitation of manganese, to affect the propertiesof the alloy.

The principle involved in this regulated heat treatment is very wellillustrated by such mag- 11;)-v

nesium-manganese alloys. Thus referring to the gure, which deals withthe precipitation heat treatment of a binary magnesium-1.4% manganesealloy (contains 1.4% manganese) at diierent temperatures for periods oftime up to 24 120 hours, it will be seenthat the yield point and Brinellhardness of the alloy at any given temperature of heat treatment tend tofollow in general a somewhat parallel change. In other words, the rateof increase or decrease in hardness is quite closely followed by acorresponding increase or decrease in yield point. Referring moreparticularly to the yield point and hardness curves for theprecipitation heat treatment at 250 C., it will be seen that thehardness increases fairly rapidly for approximately 8 hours and thenincreases very slowly up to 24 hours. The yield point, on the otherhand. increases at a uniform rate up to about 16 hours, at which pointfurther heating at the same temperature has practically no effect. Inorder to avoid any confusion as to what is meant by the term yield pointin connection with magnesium alloys, it is tobe noted that it iscommonly defined as the load at which the stress-strain curvehasdeviated 0.1% elongation from the true modulus line .(Youngs Modulus formagnesium alloys is 6,500,000 pounds per sq. in.) and the term yieldpoint as used herein is to be understood to conform to that definition.Referring next to the curves representing precipitation heat treatmentat 300 C., it is to be noted that the. yield point and hardness attainedat the end of 24 hours is much higher than was the case where 250 C. wasemployed, and that there was very little change in the yield point after16 hours had elapsed. However, during the first 16 hours the yield pointhad increased from 2500 to 4500 pounds per sq. in., in other words, theyield point had almost doubled. The Brinell hardness also was muchhigher than in the case of the 250 C. treatment for the same time. The350 C. precipitation heat treatment, on the other hand, shows that thehardness and the yield point came up quickly and reached a condition ofsubstantially uniform high properties at the end of 2 hours. Furtherheating at the same temperature had practically no effect on the yieldpoint and but very slight hardening effect. Now referring to the 400 C.curves, it will be seen that the hardness and yield point rise to amaximum point in4 about 2 to 4 hours, following which the hardnessremains substantially constant. The yield point, however, graduallyfalls back which indicates that the best temperature for precipitationheat treatment of these alloys has been passed. At

' this point it is well to note that at low temperatures ofprecipitation heat treatment indications are that the maximum propertiespossible to attain by heat treating such an alloy can be obtained onlyafter long periods of time. On the other hand,- at temperaturesapproximating 350 C. the maximum properties attainable are quicklyreached and upon further heating there is substantially no change in theyield point and hardness of the alloy. When higher temperatures, forinstance 400 C. and above, are employed, the hardness and yield pointIquickly rise to a maximum lower than that 'at 350 C. and then tend todecrease. This condition is more and more noticeable as the temperatureat which the heat treating takes place is raised. Referring moreparticularly to the 500 C. curve, as compared with the 400 C. curve, theentire curve will be seen to rise to a maximum quickly and then fallaway so that at the end of 24 hours the properties are but little betterthan those present in the casting before heat treatment. Theoretically,the cause for this is that for each temperature of heat treatment thereis a denite size to which 'accanto the particles of manganeseprecipitated canl coalesce. Where the heat treating temperature isincreased to a point above the optimum, the

size to which the particles can grow is relatively' large, in fact, solarge that part or all of the advantages of the precipitation heattreatment may be lost. In this connection, it is to be noted thatapparently the precipitated particles are elemental manganese and n'ot amagnesium-manganese compound. It is to be understood, of course, thatthis is only an attempt to explain what happens during the heattreatment and we accordingly do not wish to be limited thereby.Byreference to the curves, it is to be noted that a satisfactory rangeof operation for precipitation heat treatment of the alloys in questionwill be from 250 to 500 C. At the lower temperature the treatment will,of course, have to be carried out for a relatively long time to reachthe desired properties, while at 500 C. the heat treatment period shouldbe short in order to avoid loss in properties. A more desirable rangefor operation is between 300 and 400 C., while the preferred temperatureto be used is approximately 350 C. For example, the heat treatment of amagnesium-1.4% manganese alloy for 2 hours at 350 C. raised its yieldpoint from 2500 to 6200 pounds per sq. in. and simultaneously increasedits Brinell hardness from 32 to 38. These improvements in propertiestogether with the high corrosion resistance of the alloy make it highlydesirable for extensive commercial use. The heat treating period,however, can go as high as 40 hours or more for all except the hightemperatures whereat loss of properties occurs if the heat treatment iscarried out for too long a time. On the other hand, a heat treatingperiod as low as one hour can be used satisfactorily to increase theproperties of these alloys except for the low temperatures of heattreatment where the improvement would be slight. The range of time forthe heat treatment that will meet most requirements will be from 1 to 16hours, while the preferred time of the optimum temperature of 350 C.will be approximately '2 to 4 hours. Heat treatment at temperatures andtimes outside of these ranges where beneficial results of the typedescribed are attained are to be considered as falling within the scopeof the present invention.

From the above it is to be noted that the general principle involved isthat a magnesium alloy containing manganese in solid solution in excessof its equilibrium solubility at ordinary temperatures can be given aprecipitation heat treatment to improve its yield point and hardness.This precipitation of manganese is substantiated by microstructuralchanges and by an increase in electrical conductivity. The samecondition isol holds whether manganese is the only metal alloyed withmagnesium or whether it is only onel of several present in the alloy solong as it is present in solid solution in excess of its equilibriumsolubility. The precipitation of manganese is not limited to thetreatment of castings to obtain beneficial results but may be used withwrought metal or the metal may be given the precipitation heat treatmentand then worked if desired. At this point it is well to state that wherea" given magnesium alloy is heat treated so asto increase its hardness,the yield point of that alloy in general increases at somewhat the samerate. Accordingly, in the following examples dealing withmagnesium-cadmium-manganese alloysand magnesium-copper-manganese alloys,

alonewillbe givensincethisisasat- 1,936,550 `isfactory indicator forshowing not only increase in hardness, but increase in yield point, Analloy consisting o1' magnesium-3% cadmium- 1.4% manganese and having aninitial Brinell hardness of 34 in the as cast state was heated for 24hours at 350 C. which increased its hardness to 40. Since cadmium isperfectly soluble in magnesium and has substantially no eiect upon thehardness and yield point of the binary alloy when heat' treated, it isto be seen that the improvement in properties of the alloy was due tothe precipitation of manganese even though another metal perfectlysoluble in the magnesium was present. This was further borne out by heattreating a magnesium-10% cadmium-1.2% manganese alloy for 24 hours at350 C. which raised the Brinell hardness from 34 to 41. On the otherhand, copper is almost insoluble in magnesium. Tests on alloyscontaining this metal indicated that the same principle held with themas it did for the binary magnesium-manganese alloys. For instance, amagnesium-1% copper-1.5% manganese alloy having a Brinell hardness of 37in the as cast condition was heat treated 24 hours at 350 C. whichincreased its hardness to 40. In general copper tends to spheroidize orglobularize upon heat treatment, which condition, if anything, tends tosoften the alloy slightly rather than harden it. Accordingly, thehardening effect due to the precipitation of the manganese may have beeneven greater than was indicated, because of the possibility that it hadto overcome the softening tendency due to the spheroidizing of thecopper during the heat treatment.

A still different situation arises where a metal is alloyed withmagnesium and manganese that can be placed in solution at the sametemperature that the precipitation of manganese takes place. One suchmetal is zinc which forms an eutectic with magnesium having a meltingpoint of 344 C. In order to set forth clearly the situation involved,reference is made to the following table which is based on the averageresults of a number of tests, the same as the curves and all otherexamples given herein.

Referring to the magnesium-1.5% manganese alloy in the as cast and heattreated condition, it will be noted that the yield point was doubled byheat treating it for 24 hours at 300 C. The Brinell hardness at the sametime increased from 34 to 44. These properties are very im-y portantwhere relatively high strength commercial alloys are desired. It isfurther to be noted that the precipitation took place at 300 C. whichwill give a lower yield point than would the same period of heattreatment at 350 C., as shown by comparison of the 300 and 350 C. yieldpoint curves on Fig. 1. Referring now to the magne- Smm-3.5% zinc alloy,as shown in the above table, it will be seen that the solution heattreatment Acould be expected from the zinc.

of this alloy for a period of 16 hours at 320 C. had no eilect onhardness and a very small effect on yield point. This shows that, if analloy containingmagnesium, zinc', and manganese, having approximatelythe same amount of zinc as shown in the binary alloy, were heat treatedbelow 344 C. practically no effect on properties With this in mindreference is now made to the magnesium- 3% zinc-1.5% manganese alloy, asshown in the above table. The heat treatment of this alloy for 24 hoursat 320 C. increased the yield point from 5500 to 7800 and at the sametime increased the Brinell hardness of the alloyv from 37 to 42. It isthus to be seen that the precipitation of the manganese not onlyhardened the alloy, but had a decided effect on increasing its yieldpoint. The hardness and yieldpoint of the thus heat treatedmagnesium-zinc-manganese alloy can be further increased in appreciableamount by further heat treating at a temperature of approximately 150 to200 C. for 8 hours or more to precipitate part or all of the zinc heldin solid solution in excess of its equilibrium solubility. This showsthat the precipitation of manganese can be simultaneously accomplishedto advantage `with the solution heat treatment of another metal in thealloy where such other metal is capable of solution heat treatment atthe precipitation temperatures for manganese. It will accordingly beseen that the results of heat treatment of the magnesium-zinc-manganesealloy also bear out our discovery that, Where manganese is present insolid solution in excess of its equilibrium solubility, part or all ofthat excess can be precipitated regardless of whether the manganese isthe only ingredient alloyed with magnesium or whether other ingredientsare also present.

Binary magnesium-manganese alloys present the greatest resistance tocorrosion of any magnesium alloy known at this time, and the use ofmanganese as an ingredient in magnesium alloys containing other metal ormetals also aids in increasing their resistance to corrosion. However,the relatively low yield point and Brinell hardness has heretofore actedto prevent the more general commercial use of the binarymagnesium-manganese alloys. Accordingly, casting alloys having goodproperties, such as Dowmetal E, which is a magnesium-6% aluminum-.20%manganese- 125 alloy, have been commonly used for miscellaneouscommercial work in spite of their lower corrosion resistance. Accordingto our present invention the binary magnesium-manganese alloys can begiven a precipitation heat treatment which will raise their yield pointand Brinell hardness to a point closely approaching that of the nowcommonly used casting alloys, and at the same time such heat treatedalloy will have a better corrosion resistance than any magnesium alloynow in commercial use. The comparative properties of these alloys areset forth in the following table Dowmetal fgl'z u E u as l'l prec cast HT. 24 hrs at 300 C Yield point- 7, 800 6, 400 Brinnel hardness 47 44 145Corrosion resistance Good. The best.

From the above table and comments, it is to be noted that wehave notonly devised an entirely new method and procedure for heat treatingalloys containing manganese in solid solution in excess of Olli per centof manganese in solid soluexcess of its equilibrium solubility, but wehave obtained a product having a combination of properties heretoforeunattained by any magnesium alloy.

While we have described our invention as starting with an as cast"magnesium alloy containing manganese in solid solution in excess of itssolid solubility, we do not wish to be limited to the precipitation heattreatment of castings in the as cast form, since such castings orarticles made therefrom in some cases may be better prepared for theprecipitation heat treatment by giving them a solution heat treatment ata temperature above the precipitation heat treatment range, but belowthe melting point of the alloy. The alloy should preferably containapproximately one-half of one per cent manganese or more,`

whereas best results are obtained when the alloy contains more than oneper cent of manganese. While we. have described the precipitation heattreatment as being carried out by heating the article to the desiredprecipitation temperature and holding it there until the precipitationis partially or wholly completed, we do not wish to be limited to thatexact procedure since the cooiing of the casting after pouring or aftersolution heat treatment can be artificially retarded in a suitablefurnace or the like so as to give sufficient time for the precipitationto take place, and such procedure is to be considered as one phase ofthe present invention.

Other forms may be employed embodying the features of our inventioninstead of those explained. change being made in the method and form ofconstruction, provided the elements or steps stated by any of thefollowing claims or the equivalent of such stated elements or steps beemployed, whether produced by our preferred method or by othersembodying steps equivalent to those stated in the following claims.

We therefore particularly point out and distinctly claim as ourinvention:-

1. The process of improving the properties of a magnesium-manganesealloy in which magnesium largely predominates and which contains intion, comprising subjecting such alloy to a heat' treatment at atemperature between 250 and 500 C.

2. The process of improving the properties of a magnesium-manganesealloy in which magnesium largely predominates and which contains inexcess of 0.5 per cent of manganese in solid solution, comprisingmaintaining such alloy at a temperature between 300 and 400 C., for fromtwo to four hours.

3. The process of heat treating a magnesium base alloy having amanganese content of more than 0.5 per cent in solid solution and alsocontaining a minor proportion of at least one additional metallicconstituent from the group consisting of cadmium, copper and zinc, whichcomprises maintaining such alloy at a temperature above 250 C. but belowthe melting point ofthe most fusible constituent thereof for a period oftime to cause precipitation of manganese.

4. The process of heat treating a magnesium base alloy having amanganese content of more than 0.5 per cent in solid solution and alsocontaining a minor proportion of zinc capable of going into solidsolution in the alloy, which comprises maintaining the alloy at atemperature above 250 C. but below the melting point of thezinc-containing eutectic for a period of time to cause precipitation ofmanganese and solution of zinc.

5. The process of heat treating a magnesium base alloy having apermanent manganese content of more than 0.5 per cent in solid solutionand also containing a minor proportion of zinc capable of going intosolid solution in the alloy. which comprises maintaining the alloy at atemperature above 250 C. but below the melting point of thezinc-containing eutectic for a period of time to cause precipitation ofmanganese and solution of zinc, and then further heat treating bymaintaining the alloy at a temperature between approximately 150 and 200C. for a period of time to cause precipitation of zinc.

JOHN A. GANN.

LEO B. GRANT;

